The present invention relates to a process for the base band modulation of a data signal, as well as to the corresponding modulation apparatus and demodulation apparatus. It more particularly applies to the recording and magnetic reproduction of digital data, e.g. audio signals.
The process according to the invention is based on a (1.2) modulation, i.e. two binary values are made to correspond to an initial binary value. This modulation is of the 3PM type, which converts any sequence of n data bits into a sequence of 2.n coded bits, each represented by the presence or absence of a transition and in which two consecutive coded bits represented by a transition are separated by at least two coded bits represented by an absence of transition. In other words, if the data signal is presented to a modulator at rate T, the minimum interfront in the modulated signal is equal to 1.5T.
Several modulation processes of the 3PM type are known. From the historical standpoint, the first of these was described in the article "A new look ahead code for increased data density" by G. V. Jacoby, published in IEEE Transactions on Magnetics, Vol mag 13, No 5, September 1977, pp 1202-1204. In this modulation process, the maximum interfront (parameter an indication on the possibility during demodulation of recovering the clock signal from the modulated signal) which is received is 6T. This value is high and constitutes one of the major disadvantages of this modulation process.
Modulation processes of the 3PM type are known in which the maximum interfront is not as high. In the so-called HDM1 modulation process (HDM=high density modulation), the maximum interfront is equal to 4.5T. This interfront still starts in the modulated signal by a transition at the start of the bit interval and still finishes by a transition in the middle of the bit interval. A bit interval has a length T and is associated with a binary data item to be coded. The presence of a transition for representing the first coded bit of this binary data item corresponds to a transition at the start of the bit interval. The presence of a transition for representing the second coded bit corresponds to a transition in the middle of the bit interval. Two consecutive transitions separated by a maximum interfront, each having a fixed position (start or middle) in a bit interval, so that the detection of a sequence comprising a maximum interfront makes it possible to synchronize the clock of period T, deduced from the modulated signal, on the start of the bit intervals. However, this modulation process suffers from the disadvantage that such a sequence is rare. Thus, the synchronization of the clock cannot be checked as often as would be wished.
A type 3PM modulation process is also known, in which the maximum interfront is equal to 4T and this process is called HDM2. In this process, the maximum interfront still starts by a transition in the middle of the bit interval and terminates in the same way. However, the HDM2 process has the disadvantage of not producing a simple correspondence between a sequence of coded bits and a sequence of data bits. For example, a sequence comprising an interfront of length 3T, whose initial transition takes place at the start of the bit interval can correspond both to a sequence of three data bits of value 1 and to a sequence of three data bits of value 0. Thus, the choice between these two possibilities can only take place as a function of the context, i.e. as a function of the bits following said sequence. Thus, demodulation requires a large memory, which makes the demodulation apparatus complicated.